1. Field of the Invention
The present invention relates to a digital temperature compensating crystal oscillator. More specifically, the present invention relates to a digital temperature compensating crystal oscillator, and a method for stabilizing the frequency thereof, in which not only the vibration phenomenon caused by the conversion of analog signals to digital signals in spite of a constant abient temperature is decreased, but also the output frequency vibrations caused by the noises of analog devices are also decreased, thereby improving the stability and the reliability.
2. Description of the Prior Art
Generally, the temperature compensating crystal oscillator is constituted such that the highly stable quartz oscillator is provided with a temperature compensating means, so that the variations of the frequency due to the abient temperature can be compensated. This crystal oscillator was embodied in the analog form at first.
However, this analog type temperature compensating crystal oscillator showed a limit to meeting the miniaturization-light weight trend. At present, therefore, there is provided a digital method consisting of a couple of individual devices such as an integrated circuit and a crystal oscillator. An example of this digital temperature compensating crystal oscillator is illustrated in FIG. 1.
As shown in FIG. 1, the digital temperature compensating oscillator includes: a temperature sensing section 11 for sensing the abient temperature to output voltage signals; an analog/digital converting section 12 for converting the temperature voltages of the temperature sensing section 11 to digital data; a memory section 13 for storing oscillation compensating data for different temperatures, and for outputting oscillation compensating data, with the output data of the analog/digital converting section 12 serving as the address; a capacitor array section 14 with a plurality of capacitors connected through switching devices respectively, and with the capacitors circuitally connected in accordance with the output oscillation compensating data of the memory section 13 so as to form the required capacitance; and an oscillating section 15 including crystal oscillating elements to form a crystal oscillating circuit so as to a frequency fo, the crystal oscillating elements being connected to the capacitor array section 14.
As shown in FIG. 8a, the temperature sensing section 11 includes: a start-up circuit 311 for activating the following circuits upon supplying a power; a constant current generating section 312 for being activated by the activating circuit 311 to generate a constant current; a temperature sensing circuit 313 with its current being always constant owing to the constant current generating section 312, and with its voltage being varied by the ambient temperature; and an output amplifying circuit 314 for amplifying the voltage V1 of the temperature sensing circuit 313 to a certain level.
Here, the output temperature sensing signals of the temperature sensing section 11 are voltage signals of a certain range (e.g., 0-3 V).
As shown in FIG. 8b, the analog/digital converting section 12 includes: a reference voltage generating section 321 for outputting 2.sup.n reference voltages (where n is the number of the bits of the converted digital data) after distinguishing the ranges of the output voltages of the temperature sensing section 11; a switching section 322 consisting of a plurality of switch circuits with their one ends being connected to a respective output terminal of the reference voltage generating section 321, and with their other ends being commonly connected together, for selecting one of the output voltages; a comparing section 323 for outputting a difference between the output voltage of the switching section 322 and the output voltage B.sub.temp of the temperature sensing section 11; and an SAR section 324 for carrying out a switching to supply the reference voltages sequentially to the comparing section 323 upon supplying the power, so as to output the relevant digital data in the form of a converted data D.sub.ADC when the output value of the comparing section 323 becomes 0.
The memory section 13 may consist of an EPROM.
As shown in FIG. 8c, the capacitor array section 14 includes a plurality of capacitors C.sub.11 -C.sub.1m connected in parallel. The respective capacitors are grounded through switching devices Q1-Qm which are operated by the output data D0-DM of the memory section 13.
As shown in FIG. 8d, the oscillating section 15 includes: a crystal element X-ta1, a plurality of capacitor devices and an MOS transistor. Here, the common contact points A of the capacitor array section as shown in FIG. 8c are connected respectively to, both terminal P1 and P2 of the crystal oscillating element X-tal of the oscillating section 15.
Here, as another embodiment, the digital temperature compensating oscillator includes a digital/analog converter and a varactor diode instead of the capacitor array 14. The memory section 13 is stored with control voltages which carry out controls in relation with the ambient temperature, the control voltages being supplied to the varactor diode. Thus the voltage which is supplied to the varactor diode is varied in accordance with the sensing temperature of the temperature sensor, so that the oscillation frequency can be controlled.
At any case, however, the temperature sensing section and the analog/digital converter are necessarily provided. These two devices have the analog characteristics, and therefore, there is generated an error of .+-.2 LSB in the output data of the analog/digital converter due to the undesired noise from the designing stage.
Further, when the temperature voltages V.sub.temp which are analog signals are converted into digital signals, the temperature voltages V.sub.temp which are analog signals corresponding to the ambient temperatures are sampled at certain units to convert them into digital data D.sub.ADC as shown in FIG. 2a. Therefore, as shown in FIG. 2b, a boundary voltage Va exists which corresponds to both of the two temperature data C.sub.n-1 and C.sub.n. At this boundary temperature voltage, two digital data can be produced. Accordingly, if the output voltage of the temperature sensing section 11 is the boundary voltage Va during the digital conversion, a digital data D.sub.n-1 or a digital data D.sub.n can be outputted from the analog/digital converting section 12. Therefore, the output oscillation frequency of the oscillating section 15 can be either f1 or f2.
Therefore, when the digital temperature compensating oscillator converts the analog temperature voltages to digital data, if the ambient temperature lies at the boundary of the resolution of the analog/digital converter, the sampled temperature data can be continuously varied, and therefore, the output frequency is seriously influenced.
In order to solve the problem of the boundary value, Motorola Pendulum-LV and Chronos-LV User's Guide proposes the use of an anti-dither circuit. In this vibration eliminating circuit, only if the temperature data which have been continuously obtained by a certain number of times through the analog/digital converter are all different from the previously stored temperature data, then a new temperature data is outputted. In this method, therefore, only if the temperature sampled data which have been continuously obtained more than a certain number of times are different from the previous temperature, then the frequency compensation with respect to the temperature variation is realized.
However, in this oscillator of the Motorola company in which the above described method is applied, there is adopted a temperature compensating code of 4 bits. Therefore, no influence is received from the variations of the adjacent temperature codes during the sampling. However, if the data hits are increased to more than 10 bits to improve the precision in accordance with the request of users, then the variations of the temperature codes due to the self characteristics of the analog temperature sensing section and the analog/digital converting section cannot be prevented.
Particularly, no matter how perfectly an analog/digital converter may be designed, basically there is an error of .+-.1. Further, the inherent error of the temperature sensor cannot be ignored, and even if the above described conventional method is applied, the vibrations of the output frequency at a constant temperature cannot be avoided.